Species dispersal and community assembly facing the Atlantic Meridional Overturning Circulation

Species dispersal is more complex in the ocean than on land because most marine biota have early life stages that are passively transported by currents within the plankton. By being spawned and recruiting in distinct local communities, these drifting organisms interconnect distant marine systems into large metacommunities. Consequently, the origin of the organisms recruiting to a particular local community has great influence on its biodiversity. The strong circulation governing western boundary current systems poses challenges for species recruitment and the assembly of local communities. We have investigated and highlighted this through a series of high-resolution (1/36°) community-based Lagrangian dispersal modelling experiments conducted along the Tropical Southwestern Atlantic, which is part of the Atlantic Meridional Overturning Circulation (AMOC).
In western boundary current systems, the strong currents flowing along the coast are highly advective, and organisms spawned along the coast are rapidly transported away. Therefore, coastal and neritic species face a challenge in recruiting to the suitable habitats present along these systems. The western boundary currents also spread open ocean waters and their associated fauna over the continental shelf. The continuous arrival of allochthonous species, overlapping local retention, results in the mass-effect assembling archetype, leading to homogeneous local communities dominated by the best dispersers, excluding autochthonous species and, consequently, reducing local and regional biodiversity.Despite this dominant assembly pattern, stochastic and seasonal fluctuations around the western boundary current such as eddies, meanders and countercurrents, can create hotspots of autochthonous species retention. Specifically for the Tropical Southwestern Atlantic, these fluctuations are caused by the interaction of currents and topography and the seasonal variability of the intertropical convergence zone. At these hotspots, species in each local community are sorted according to their ecological niches and interactions with other autochthonous species (species sorting assembling archetype), resulting in heterogeneous local communities and higher regional biodiversity. Similarly, when islands and seamounts are present off western boundary current systems, as we observed along the Fernando de Noronha Ridge, strong epipelagic westward currents pose challenges to the retention and recruitment of organisms. Particularly if no other shallow water source systems is present to the east. However, we saw that the diel vertical migration  allow organisms to reach depth strata with reduced current speeds or even undercurrents flowing in the opposite direction to the surface. This condition increases the retention of autochthonous species and the possibility for organisms spawned over the continental shelf to reach these offshore systems, either directly or through stepping-stones.The dispersal patterns observed in the Lagrangian experiments align with the few already known spatial patterns of biodiversity distribution along the Tropical Southwestern Atlantic and provide important insights for regions and taxa for which knowledge is limited or absent. These results also provide elements for the proper definition of biodiversity conservation and management strategies and for a better understanding of the processes regulating community assembly in highly advective systems around the globe.